The field of this invention is processes for upgrading petroleum feedstocks, particularly heavy feedstocks having high metal and Conradson carbon residue contents.
In refining petroleum feedstocks to intermediate or final petroleum products, the feedstocks are often subjected to one or more catalytic processes such as hydrodesulfurization, fluid catalytic cracking, heavy oil cracking, or the like. However, if heavy feedstocks are fed directly to such processes, problems can be encountered.
Heavy feedstocks are rich in coke precursors as evidenced by the typically high Conradson carbon residue content of these feedstocks. When the feedstocks are fed to the catalytic refining processes operated at elevated temperatures, an undesirably high level of coke formation occurs in the catalytic reaction zone. This coke tends to deposit on the catalyst and reduce the catalytic activity for promoting the desired reaction. In many instances, the adverse effects of the coke can be reduced to a tolerable level by periodic regeneration of the catalyst. However, excessively frequent catalyst regeneration requirements can adversely affect the economics of the catalytic refining processes to the point that they are no longer commercially acceptable for processing the heavy feedstocks.
Heavy feedstocks also typically contain high levels of heavy metals, principally vanadium and nickel. These metals are present in the heavy feedstocks in several forms including organometallic complexes such as metal prophyrins and their derivatives. If present in sufficient quantities, the metals adversely affect a wide variety of catalytic reactions including hydrodesulfurization, fluid catalytic cracking, heavy oil cracking, hydrocracking, and the like. If the feedstocks with high metal contents are passed to the catalytic reaction zone of such processes, the metals from the feedstocks deposit on the catalyst and reduce the desired catalytic activity and selectivity. The metals are themselves catalysts for dehydrogenation reactions which tend to increase hydrogen and coke formation at the expense of the desired petroleum products.
To reduce the adverse effects of the metals on the catalyst, many processes periodically withdraw catalyst from the catalyst inventory and replace the withdrawn catalyst with fresh catalyst in an effort to maintain the overall amount of metals in the catalyst inventory at a tolerable level. This replacement of catalyst keeps the total amount of metals in the reaction zone at equilibrium. The metals level in a reaction zone remains at equilibrium when the weight of metals removed by withdrawal of catalyst equals the weight of metals entering with the feedstock. The withdrawn catalyst is known as equilibrium or equilibrated catalyst, and that term is used herein to define such catalysts. Withdrawing equilibrated catalyst and replacing it with fresh catalyst is effective in many instances, especially when combined with catalyst regeneration steps. However, if the level of heavy metals in the feedstock is sufficiently high, excessively frequent catalyst replacement and regeneration rates are required and conversion of the high metal feedstocks becomes uneconomical.
A large portion of the heavy metals, coke precursors, and sulfur of heavy feedstocks are included in the asphaltic fraction of the feedstock. In particular, the metals tend to be complexed with the feedstock asphaltenes. As used herein, the term "asphaltic fraction" shall mean asphaltenes, carbenes, carboids, and closely associated resins and very heavy oils. Asphaltenes are isopentane insoluble materials which constitute a part of the asphaltic fraction. Carbenes and carboids are trichloroethylene insoluble materials which also comprise a part of the asphaltic fraction.
The asphaltenes, which are contained in the feedstock as a colloidal solution in resins, have very large molecules with fused aromatic rings, making the asphaltenes relatively difficult to convert to desired, lighter petroleum products. Conversion of the asphaltenes is made even more difficult by the fact that as they are subjected to the heat of a preliminary distillation step typical of many refining processes, the asphaltenes tend to flocculate and polymerize.
The difficulties associated with converting heavy feedstocks can be substantially reduced by utilizing a heavy oil cracking process, also known as the HOC Process. The HOC Process and its operation are well known to those skilled in the art and are described in U.S. Pat. No. 3,862,899 and "Heavy-Oil Cracking Boosts Distillates" by J. A. Finneran, J. R. Murphy and E. L. Whittington, The Oil and Gas Journal, Vol. 72, pp. 52-55, Jan. 14, 1974. The HOC Process differs from ordinary gas oil fluid catalytic cracking processes most notably in that the HOC Process handles feedstocks with much higher Conradson carbon residue contents than can be accommodated by gas oil FCC units and the HOC Process handles feedstocks with higher metal contents than can be accommodated by gas oil FCC units. Nevertheless, there are presently available feedstocks whose very high metal and carbon residue contents make them economically unattractive feedstocks for conversion even with the HOC Process.
Adverse supply and cost factors associated with light, easily refinable crude oils have, however, made it increasingly apparent that heavy crudes will have to be refined to satisfy the ever increasing demand for petroleum products. As a result, significant efforts have been directed to processing heavy feedstocks.
Four of the most notable processes representative of the previous efforts to process heavy feedstocks include the HOC Process described above, residual desulfurization, solvent deasphalting, and coking. Residual desulfurization is a catalytic process aimed primarily at producing low sulfur fuel oils. When high metal feedstocks are used in this process, it is sometimes necessary to initially treat the feedstock to remove metals in order to achieve acceptable catalyst life for the desulfurization process. Residual desulfurization is quite expensive, and this is one of the factors leading some skilled in the art to the conclusion that flue gas desulfurization is preferable to residual desulfurization. Solvent deasphalting, a process which uses solvents to precipitate an an asphaltic fraction and recover a better quality oil for further processing, is also quite expensive. Difficulty in handling the precipitated asphaltic fraction is also one of the drawbacks to this process. Coking, a thermal cracking process, produces a coke product which is often very high in sulfur content, and therefore, hard to market. In some cases, the coke is converted to low heating value gas by partial oxidation.
Specific illiustrative examples of efforts to process heavy feedstocks include the process disclosed in the U.S. Pat. No. 2,891,005. In that patent, high boiling oils are said to be freed of metal contaminants and compounds which form stack solids when the oils are burned. To remove the contaminants, the feed is contacted in the presence of hydrogen with a hydrogenation catalyst at a pressure of from about 400 psig to 3000 psig and at a temperature of about 750.degree. F to about 825.degree. F. The patent does not indicate what effect, if any, this treatment has on the boiling range of the feed. The patent does state, however, that the contaminants present in the feed form into what is termed a "micro-coke", a modified naphtha insoluble material which is separable from the hydrogenation zone effluent by filters operated at high temperatures, centrifuges, centrifugefilters, or the like. The micro-coke purportedly does not significantly foul the hydrogenation catalyst in the reaction zone; however, no mention is made of the life of the hydrogenation catalyst.
Another patent, U.S. Pat. No. 3,362,901, discloses a two-stage hydrogenation process of reduced crudes for removing a portion of the asphaltenes from the reduced crudes. In the first stage, the feedstock is contacted with either a hydrogenation catalyst or an inert material in the presence of hydrogen and at a pressure of from about 100 psig to about 2500 psig and a temperature of about 600.degree. F to 900.degree. F. A portion of the asphaltenes of the feedstock are said to agglomerate over the first stage catalyst in such a manner that asphaltenes are separable from the effluent of the first stage by hot filtering or by flashing off the more volatile hydrocarbon materials of the effluent. The second stage hydrogenation treatment is conducted over a more active catalyst than that of the first stage in order to achieve the desired product of the process.
Two additional patents disclose methods of processing reduced crudes which employ both hydrocracking and hydrodesulfurizations steps. U.S. Pat. No. 3,380,910 discloses a process in which a residuum is contacted in the presence of hydrogen with a hydrocracking catalyst at temperatures from about 300.degree. C to about 550.degree. C. After gas-liquid separation, the liquid phase of the hydrocracker effluent is subjected to atmospheric distillation and then vacuum distillation. A final liquid residual containing tars and asphaltenes is obtained from the bottom of the vacuum distillation unit and is passed to a partial oxidation process for the production of hydrogen. The lighter hydrocarbons are passed to a hydrodesulfurization unit for further processing.
In a somewhat similar manner, U.S. Pat. No. 3,825,485 discloses a process in which a petroleum feedstock is hydrocracked and then desulfurized. Feedstock is first contacted with a hydrocracking catalyst in the presence of hydrogen at a temperature of from about 750.degree. F to about 950.degree. F and at a pressure of between 500 psig and 5000 psig. The hydrocracking unit is operated to obtain a 25-70 percent conversion of the 1000.degree. F plus portion of the charge stock to materials boiling below 1000.degree. F. The effluent of the hydrocracking zone is then cooled to a suitable temperature for introduction to the second stage desulfurization zone. The patent states that since some light hydrocarbons are formed in the hydrocracking step, there may be a tendency with some feedstocks for asphalt to precipitate as a result of the temperature reduction and the presence of the lighter hydrocarbons. The patent teaches that to avoid such precipitation and the resulting plugging of the apparatus, the cooling is preferably accomplished by the addition of an aromatic rich fraction which is introduced at a temperature sufficient to effect the desired temperature reduction of the hydrocracking effluent. The subsequent mixture is then passed to the hydrodesulfurization zone for final processing.
The processes described generally and specifically above each have limitations with respect to the extent to which heavy feedstocks are upgraded by the processes or with respect to the economic attractiveness of the processes. Accordingly, there has existed a need for a process which will economically upgrade heavy feedstocks.